How Can I Iterate Over a Parameter Pack in C Using Pack Expansion?

Variadic Templates: Unraveling Pack Expansion for Iteration

Variadic templates grant remarkable expressiveness in C , but venturing into their realm can reveal unforeseen complexities. One such challenge arises when attempting to iterate over a parameter pack using pack expansion. Consider the following code:

template<typename T>
static void bar(T t) {}

template<typename... Args>
static void foo2(Args... args)
{
    (bar(args)...);
}

int main()
{
    foo2(1, 2, 3, "3");
    return 0;    
}

Upon compilation, this code encounters an error: "'args': parameter pack must be expanded in this context." The issue stems from the inability to directly expand a parameter pack in a function call.

The solution lies in utilizing pack expansion within a braced-init-list, a context that explicitly allows such expansion. By placing the expansion inside the initializer list of a dummy array, we can force the desired behavior:

template<typename... Args>
static void foo2(Args &amp;&amp;... args)
{
    int dummy[] = { 0, ( (void) bar(std::forward<Args>(args)), 0) ... };
}

This revised code embraces a meticulous approach to ensure correct evaluation:

• Guard Against Empty Expansion: An empty Args will result in a 0-length array, which is illegal in C . To prevent this, we ensure the array contains at least one element.
• Perfect Forwarding: We use perfect forwarding (std::forward) to avoid potential copies and preserve potentially-const qualifiers.
• Neutralize Return Type: We cast bar()'s return value to void to leverage the built-in comma operator, ensuring left-to-right evaluation.

In C 17, fold expressions provide an even more compact solution:

((void) bar(std::forward<Args>(args)), ...);
```

This approach guarantees the desired left-to-right expansion unequivocally.

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